Lens-grid system for electron tubes

ABSTRACT

An electron tube such as an x-ray tube has an anode target spaced from an electron beam emitting structure which include a cathode, focusing electrode and a control electrode. The control electrode surface nearest the anode is contoured to conform with selected equipotential values in the electrostatic field between the cathode and anode which equipotential is a predetermined percentage of the total cathode to anode voltage. When the control electrode is operated at a corresponding positive potential, the electrons follow certain trajectories and focus on the anode. Means are provided for varying the control voltage through a range from beam cut-off voltage to various positive voltages that permit control of the focal spot size regardless of the selected beam current and selected anode voltage.

United States Patent 1191 Heiting et a1.

[ 1 LENS-GRID SYSTEM FOR ELECTRON TUBES [75} Inventors: Robert F.Heiting, Milwaukee;

Edward T. Rate, Jr., Mequon, both of Wis.

[73] Assignee: General Electric Company,

Schenectady, N.Y.

[22] Filed: May 3, 1974 [21] Appl. No.: 466,728

[52] US. Cl. 250/403; 250/405; 313/452 [51] Int. Cl. H056 1/30 [58]Field of Search 250/403, 404, 405, 399, 250/401, 396; 313/452, 454, 453

[56] References Cited UNITED STATES PATENTS 1,203,495 10/1916 Coolidge250/401 2,053,792 9/1936 Huppert et a1... 250/401 2,173,165 9/1939Headrick 313/454 2,215,426 9/1940 Machlett 250/403 2,308,800 l/l943Anderson 313/452 2,518,472 8/1950 Hell 313/454 2,665,384 1/1954 Yockey250/396 llllIlll Ill 11 3,916,202 14 1 Oct. 28, 1975 2,793,282 5/1957Steigerwald ..250/39s 3,141,993 7/1964 Hahn 2501396 3,614,520 10/1971Coleman .1 250/396 [57] ABSTRACT An electron tube such as an x-ray tubehas an anode target spaced from an electron beam emitting structurewhich include a cathode, focusing electrode and a control electrode. Thecontrol electrode surface nearest the anode is contoured to conform withselected equipotential values in the electrostatic field between thecathode and anode which equipotential is a predetermined percentage ofthe total cathode to anode voltage. When the control electrode isoperated at a corresponding positive potential, the electrons followcertain trajectories and focus on the anode. Means are provided forvarying the control voltage through a range from beam cut-off voltage tovarious positive voltages that permit control of the focal spot sizeregardless of the selected beam current and selected anode voltage.

8 Claims, 8 Drawing Figures US. Patent Oct.28, 1975 shw 1 of2 3,916,202

Sheet 2 of 2 332501 Cum/i N m ZFZMEE mim m Emo SPOT WIDTH MM FIGSLENS-GRID SYSTEM FOR ELECTRON TUBES BACKGROUND OF THE INVENTION Thisinvention pertains to electron tubes and will be exemplified in an x-raygenerator tube.

In connection with x-ray generators used for medical diagnosis it iscustomary to provide for selecting the operating characteristics such asthe anode to cathode voltage, the electron beam current, the focal spotsize and the conduction or exposure time interval. For high speed, shortduration exposure technics such as cineradiography, the x-ray tube isusually provided with a focusing electrode to shape the electric fieldaround the electron emitting filament for controlling the beam crosssection so that a suitable focal spot is formed on the anode target. Thecontrol electrode is usually operated at cathode potential for fullelectron beam current and is sometimes biased negatively when electronbeam cutoff is desired. The control electrode undesirably reduces theelectric field strength in the vicinity of the cathode filament and anelectron space charge which limits maximum available beam currentresults. Here tofore it has been the practice to compromise maximumobtainable beam current, minimum bias cutoff potential and focal spotsize. This results in part from ased to cutoff.

Most rotatinganode diagnostic x-ray tubes have one large and one smallfilament which are switched on selectively to produce a single focalspot size for each filament. Biasing power supplies for focusingelectrodes are available which permit reduction of the width dimensionof the focal spots but these have the disadvantage of reducing theavailable tube current by biasing the thermionic cathode toward beamcurrent cutoff when a negative voltage is applied to the electrode forreducing focal spot size. Up to the present, continuously variable focalspot widths have not been practical nor available because of the widevariations in available beam current that would result.

In reference to prior art x-ray tubes, the difficulty of controllingfocal spot size and beam cutoff with reasonably low bias voltages hasplaced a limitation on per- I missible beam current. Generally x-raytubes are oper ated in a temperature or emission limited mode which isto say that beam current is controlled by adjusting filament currentand, hence, filament temperature. I-Iow ever, the geometry of thefocusing electrodes, the electrostatic field configuration and the highrequired biasing'potentials prevailing in prior x-ray tubes militatedagainst obtaining beam current intensities commensurate with theemissivity limits of the filament.

SUMMARY OF THE INVENTION A general object of the present invention is toovercome the above noted disadvantages and to provide a more efiicientand more readily controllable electron emission device such as an x-raytube.

Further objects of this invention are' as follows:

To permit varying the focal spot size in an x-ray tube for any practicalvalue of target anode voltage without markedly affecting tube current;

To enable maintaining the focal spot size constant when tube current isheld constant for any practical value of anode voltage; I

To permit switching a high voltage x-ray tube on and off at a highratewith a low control or bias voltage;

To significantly increase beam current over that which is obtainable atcorresponding cathode temperatures in prior art tubes;

To enable use of positive voltages on the control electrode or grid ofan x-ray tube without substantial electron current flow in the controlelectrode circuit;

To provide a grid which is not imaged in.the focal spot on the x-raytube anode;

To substantially vitiate the effect of space charge in the vicinity ofthe emitting cathode in an x-ray tube; and

To provide a new cathode support structure which overcomes theheretofore experienced misalignment and distortion that results fromcyclical heating and cooling of cathode structures.

The invention may be characterized briefly as a new lens-grid system foran electron tube such as an x-ray tube. The new lens-grid system orcathode structure comprises a metal focusing electrode in which acupshaped recess is formed. The recess has a pair of spaced apart bottomslots in which there. are individual filaments, one for obtaining a highrange of beam currents and the other for obtaining arelatively lowerrange of beam currents. The filaments may be energized alternately.There is the usual anode or target on which the electron beam impingesin a focal spotfrom which xrays emanate. This much of the constructionof the tube is known. a

In a tube of the character just described, a high gradient electricfield is produced between the cathode and the target anode when thelatter is energized at high voltages. In diagnostic systems the anodevoltage may range'from peak kilovolts (pkv) to l50pkv though sometimesthe range extends to even lower and higher voltages. As is known,equipotential lines or surfaces of the electric field can be measuredand determined or plotted between the cathode and the anode. Theconfiguration of the various equipotentials and the potential gradientgoverns the focusing efiect of the field. To obtain full electron beamcurrent, the focusing cup electrode is established at the same potentialas the cathode filament in which case the beam will focus on the targetin a spot having predetermined width. When it is desired to cutofi' thebeam, the potential of the focusing electrode is made very negative withrespect to the filament.

In prior art tubes, an additional control electrode or grid is sometimesinterposed between the focusing electrode and the target anode to obtainfurther control over electron beam current. A major disadvantage of thisis that if grid biasing potential goes positive with respect to thecathode excessive grid current flows which results in overheating thegrid. I-Ience, prior control electrodes were usuallyoperated at somenegative voltage with respect to the filament to avoid grid current inwhich case beam current could not be drawn up to the temperaturegoverned limits of emissivity of the focal spot quality because the gridwas imaged in the focal spot.

In the new lens-grid system described herein, a control electrode islocated near the focusing electrode. The control electrode surface mostremote from the filament in the anode direction is made coincident withand in substantial contour conformation with a particular equipotential.In its preferred form, an equipotential representing a predeterminedpotential, such as a relatively low percentage of the cathode to anodevoltage, is chosen. When this low percentage of the cathode to anodevoltage is applied to the new control element in reference to thecathode, the electron beam from the chosen filament focuses on the anodewith a predetermined focal spot size since the trajectories of theelectrons are not changed. Variations of the potential on the controlelectrode, however, permit enlarging or decreasing the width dimensionof the beam and focal spot without requiring any anode potential changeand without significantly affecting the total electron beam current.When the control electrode is made about half as negative as washeretofore required, the beam current is completely cut off. The controlelectrode has no grid wires which could be imaged in the focal spot.

How the foregoing and other more specific objects of the invention areachieved will be evident in the ensuing description of an illustrativeembodiment of the invention taken in conjunction with the drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section view of arotating anode x-ray tube with some parts omitted and which incorporatesthe new lens-grid system;

FIG. 2 is a plan view of the cathode structure or Iensgrid system takenalong the line 2-2 in FIG. 1;

FIG. 3 is a section taken along the line 33 in FIG.

FIG. 4 is a section taken along the line 4-4 in FIG.

FIG. 5 is a section taken along the line 5-5 in FIG.

FIG. 6 is a schematic representation of the new lensgrid structure andshowing the configuration and position of some of the equipotentiallines;

FIG. 7 is a graph showing the relationship between the positive biaspotential on the control electrode versus the focal spot width for twodifferent focal spot sizes in a tube which uses the principles of theinvention; and

FIG. 8 is an electric circuit diagram for an x-ray tube in which theinvention is incorporated.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows an x-ray generator ortube in which the new lens-grid structure is used. The tube comprises atarget or anode 10 which is mounted on a rotor 11 that is internallyjoumaled for rotation on a stem 12. An extension 13 affords a place forconnecting an anode voltage supply line. Anode 10 has a beveled electronimpact surface 14 on which an electron beam is focused in a spot fromwhich x-rays emanate. Spaced from target 10 is the new cathode structureor lens-grid which is generally designated by the number 15. The cathodestructure 15 is supported on a mounting device 16 that has a baseannulus 17 whose edges are sealed into the ends of an annular reentrantglass section 18 that forms part of the evacuated x-ray tube envelope.The midregion of the envelope comprises a thin metal shell 19 that isprovided with a thin window 20, which may be metal or glass, throughwhich the useful x-ray beam emanates from the focal spot on targetsurface 14. Metal section 19 is sealed at one end into a glass tubularelement 21 which joins sealingly at 22 with a mounting ferrule 23 forthe rotating anode structure. The conventional field coils for inducingrotor 11 to rotate are omitted from FIG. 1.

Refer now to FIGS. 2-5 for a more detailed description of the cathodestructure or lens-grid system 15.

In FIG. 3 it is apparent that the structure comprises a metal body 30 ofsubstantially circular configuration. Body 30 is herein called a firstfocusing electrode. Focusing electrode 30 may experience as high asabout 800 C during tube operation so it is desirable to make the body ofa suitable temperature resistant metal such as nickel or molybdenumalthough other suitable refractory metals may be used. The upper portionof electrode 30 is provided with a pair of slots 31 and 32. Slot 31accommodates a large or heavy current filament 33 and slot 32accommodates a smaller lower current filament 34. The filaments areelectrically isolated from the focusing electrode. The slots may bespaced symmetrically from a diametral line that runs across the top ofelectrode 30 in a direction normal to the drawing where the numeral 35is affixed in FIG. 3. Slot 31 has diverging stepped walls 36 and 37whose configuration has an effect on electrostatic field andequipotential line distribution in the vicinity of filament 33.Similarly, slot 32 has divergent stepped side walls 38 and 39 whichserve the same purpose. The open space between the topmost projections40 and 41 in electrode 30 is characterized as a focusing cup or focusingelectrode. In a single filament tube the diverging walls of the focusingcup recess would be symmetrical to the filament.

The structure is also provided with a second electron permeablelens-grid electrode that is generally designated by the number 45 and issupported from electrode 30. Electrode 45 is preferably cylindrical andhas an annular side wall 46 and substantially planar portion 47. Portion47 has a diametral or transverse slot in it and the sides or edges 48and 49 of this slot diverge outwardly from each other with a particularcontour or configuration that will be discussed in more detail later.The slot is generally designated by the number 50. A small rodlikeelement 51, which in this case is semicircular in cross section, extendslengthwise of the slot 50 and along its midline. The rod 51 should be asflat and thin as possible but it is made semicircular to obtainrequisite strength. As can be seen in FIG. 2, rod 51 has one end 52spotwelded to annulus 46 and has its other end 53 overlaid by a metalstrip 54 whose opposite ends are also spotwelded to wall 46. Thus, rod51 is restrained from being removed but it can expand lengthwise understrip 54 when it is subjected to intense heat without developinginternal stresses that would otherwise deform it. Rod 51 participates inestablishing the electrostatic field configuration involved in focusingand controlling each of the separately selected electron beams.

It is evident in FIGS. 3 and 4 that the annular wall or rim 46 ofcontrol electron 45 is in concentric spaced relationship with theradially extending portion 55 of electrode body 30. Thus, an annular gap56 is formed between portion 55 and wall 46 to isolate the controlelement 45 electrically from electrode 30. There are four equiangularlyspaced ceramic pins 57-60 extending from holes 61 in electrode 30. Thesepins enter holes 62 in the control electrode annular wall 46 and thepins are constrained to remain in the holes by a surrounding circularmetal band 63 which is inset into the outer periphery of wall 46 andfastened thereto such as by welding. The pin support constructionafi'ords control electrode 45 and electrode 30 an opportunity to expandand contract thermally without generating undue internal stresses ineither of these parts. An advantage of the construction is that itobviates the need for matching the thermal expansion coefficients of thecontrol element and focusing cup body in which case a wider choice ofmetals for making these components is allowed.

As can be seen in FIG. 4, electrical connections to filament 34 are madethrough ceramic bushings 67 and 68 through which lead wires 69 and 70,respectively, extend to join the ends of filament 34. The leads forlarge filament 33 are also extended through a pair of ceramic bushingsone of which, 71, is visible in FIG. 3.

Refer now to FIG. 6 for a discussion of the functional features of thelens-grid system. This FIGURE shows schematically the geometricalrelationship of the focusing electrode 30, the control electrode topsurface 45, the electrode recess dividing rod 51 and the anode 10 withits electron impact target surface 14. Filaments 33 and 34 are shown intheir slots 31 and 32, respectively. Some of the electrostaticequipotential lines between the cathode structure and the anode are alsoillustrated. Note that the longitudinal axes of the filament wire coils33 and 34 are in parallelism with electron impact surface 14 of anode 10in FIG. 6. It is to obtain this parallelism that the cathode structurein FIG. 1 has its face nearest target surface 14 at an angle withrespect to the longitudinal axis of the tube. In FIG. 1 the longitudinalaxes of filaments 33 and 34, not visible in this FIGURE, extendgenerally radially from the center of the x-ray tube or in the radialdirection of the target. If the filament axes and target surface 14 aregrossly nonparallel electrostatic field and equipotential symmetry wouldbe lost when control voltage is varied. Dividing rod 51 is also presentin FIG. 6. As suggested earlier, rod 51 should preferably be flat and asthin as possible so as to lie in a plane coincident with the plane ofthe equipotential surface which represents a potential equal to apredetermined percent of the cathode to anode potential.

In FIG. 6 the solid equipotential lines 80-86 are those that exist whenthe lens-grid 45 is biased to a potential equal to the equipotentialvalue of the line that coincides with the contour of its surfaces 48 and49. As explained earlier, when this condition exists, the electron beamemitted from either of the cathode filaments 33 or 34 will focus onanode surface 14 with a predetermined spot size and the trajectory ofthe electrons will not be affected by the selected equipotential. Theparticular equipotential plot shown in FIG. 6 represents the case. where5% of the cathode to anode voltage is applied between lens-grid 45 andthe filaments 33 and 34. The values of the equipotential lines in termsof percent of cathode to anode voltage are also applied to solid lines80-86. Equipotential lines between of cathode to anode voltage and l00%are omitted from the plot in FIG. 6. The omitted lines gradually becomemore straight than the 20% line 86 and eventually become substantiallyparallel to the anode surface 14 and to each other. The equipotentialplot has been obtained by methods that are familiar to those skilled inthe electron tube arts and need not be described in detail.

In FIG. 6, the lens-grid 45 could also be formed to have its contouredsurfaces 48 and 49 and the straighter surfaces confluent therewith tocoincide with some other equipotential line such as the 7.5% line orsomething under 5% such as the 4% or 3% lines which are not shown. Inany case, within limits, it is necessary to apply to control electrode45 a bias potential equal in value to the equipotential to obtain theresult that the trajectories of the electrons traversing the field willnot be altered by the equipotential surface coincident with the surfaceof the lens-grid. Practically, it is desirable to locate controlelectrode 45 on an equipotential corresponding in value with a biaspotential in the range of l to l5% of the applied cathode to anodevoltage. Choosing a higher than 15% equipotential would requireoperating at unduly high bias voltage.

In FIG. 6 the normal 6% equipotential existing when 5% bias voltage isapplied to lens-grid 45 is shown partially as a dash-dot-dot line 87.The purpose is to illustrate what happens when the bias voltage on thelensgrid is changed to 6% of the cathode to anode voltage in a casewhere the contour of the lens-grid coincides with the 5% equipotential.Thus, when a 6% bias voltage is applied to the lens-grid, the 6%equipotential shifts to partially follow the contours of surfaces 48 and49 which are equipotential and to further assume the shape of thedash-dot line 88. The actual result is that cathode filament 33 comesunder a more positive influence. In other words, the field strengthpresented to the emitting filament 33 or 34 is increased and an increasein the maximum available beam current results provided the filament isnot being operated in an emission or temperature limited mode. However,x-ray tubes are usually operated in a filament temperature limitedemission condition and there is usually substantial space charge but theoperative effect of increasing the positive potential on lens-grid 45according to the invention permits changing focal spot width withoutsignificantly altering target current or target voltage which is adesired objective of the invention.

An important aspect of having the contour of the lens-grid electrode 45coincide with a selected equipotential is that positive control voltagesmay be applied to the lens-grid 45 without substantial electron currentflowing to it. Thus, in accordance with the invention, beam current isnot diminished as a result of grid control nor is consequential gridheating experienced as was the case in prior art grid controlled tubes.By way of example, in an x-ray tube constructed in accordance with theinvention, a grid current of only 600 microamperes was measured when thetube was conducting 1,800 milliamperes of electron current between thecathode and anode even though the lens-grid was positive with respect tothe active filament.

Besides permitting beam width control by varying the positive biaspotential on lens-grid 45, the new lens-grid system permits completecurrent cutoff from the x-ray tube when it is operating at high cathodeto anode potentials. As was implied earlier, it is believed thatheretofore the largest focal spot size with cutoff bias capability wastypically 1.2mm size since switching bias voltages of greater than 5kilovolts in the millisecond exposure times was considered impractical.Typically in the prior art, at cathode to anode voltages of 75 kvp, anegative bias voltage of kilovolts was required to cutoff a focal spotsize of 1.2mm. With the present invention, complete beam current cutoffis obtained with 3.5 kilovolts negative bias on the lens-grid 45 withrespect to the filament at a cathode to anode voltage of 150 kvp for anyfocal spot size. Moreover, to typify the effect of positive grid controlin accordance with the invention, a variable spot width capability ofless than 0.6 to greater than 2.0mm in a single tube was obtainable.Spot width could be varied with unsubstantial change in beam currentover most of the useful operatmg range.

FIG. 7 is a graph of positive bias potential applied to the lens-grid interms of percent of voltage between the cathode and anode of the tubeversus spot width in millimeters in which tube the new lens-grid systemwas used. Note that the large spot width produced by the beam fromfilament 33 was varied from under 2mm to over 3mm at constant cathode toanode voltage and constant beam current. The small focal spot which isrepresented by line 92 was varied from about 0.5 to over 2mm where thebias potential was changed from about 3 to 5.5% of cathode to anodevoltage.

The means for selecting one or the other of the filaments are not shownbut it will be understood by those skilled in the art that when a highcurrent beam is desired such as for various radiographic technics, largefilament 33 will be energized. When low beam current is desired such asfor fluoroscopy, filament 34 will be selectively energized to theexclusion of the other filament. It will also be understood that theequipotential plot shown in FIG. 6 and discussed primarily in respect touse of filament 33 is symmetrical so that what has been said appliesequally as well to use of filament 34.

It is also of interest that with a tube made in accordance with thepresent invention, 3,000 milliamperes of beam current were obtainable at65 kvp anode voltage at filament temperatures equal to those at whichabout half of the same beam current was obtained in prior art tubes. Thereason for this is that the positively biased lens-grid in accordancewith the invention enables greater field influence in the vicinity ofthe filaments in which case the space charge effect is more nearlyvitiated and this limitation in prior art tubes is removed.

FIG. 8 is a schematic diagram for illustrating how an x-ray generatorusing the new lens-grid is operated. Only one cathode or electronemitter 33 is shown for the sake of simplicity since the electricalconnections are the same for the other selectable emitter 34. The targetanode is marked 10, the focusing cup electrode is marked 30 and thelens-grid electrode is marked 45 as they are in the previously discussedfigures. Two bias voltage sources 102 and 103 are shown in block form.These sources are connected to the electrodes through a double pole,double throw switch 104 which is symbolized as a mechanical switchalthough various types of switches may be used. Potential foraccelerating electrons from cathode 33 to target anode 10 may be appliedacross terminal 105 and 106 with the potential on terminal 105 beingnegative and below ground potential by as much as the absolute value ofthe potential on terminal 106 is positive or above ground potential. Aground or midpoint potential terminal is shown. Midpoint is obtainedfrom the center tap of the high voltage transformer which is not shownsince it is of a known type. The metal shell 19 of the x-ray tube shownin FIG. 1 may be established at ground potential.

In FIG. 8, when switch 104 is in the state in which it is depicted, biassource 102 makes lens-grid 45 positive with respect to cathode 33 andfocusing electrode 30 and full electron beam current flows to anode 10.Adjustment of bias source 102 permits attainment of different selectedfocal spot sizes in accordance with the level of positive potential onlens-grid electrode 45 as described in detail hereinbefore.

Switching switch 104 to its alternate state applies voltage from biassource 103 to lens-grid 45 and focusing electrode 30 at the same time sothat these electrodes are both negative with respect to emitter 33 andelectron flow to anode 10 is cut off. Typically, a negative potential ofabout 3500 volts will effectuate cutoff even when the anode 10 tocathode 33 potential is 150 kvp.

Although an illustrative embodiment of the new lensgrid system and itsoperating characteristics have been described and a new mode of mountinga lens-grid has been described in considerable detail, such descriptionis intended to be illustrative rather than limiting for the inventionmay be variously embodied and is to be limited only by interpretation ofthe claims which follow.

We claim:

1. An x-ray generator comprising:

a. cathode means including electron emitter means whose electronemission is limited and controlled primarily by the selected temperatureof said emitter means and whose electron emission available for formingan electron beam is subject to further limitation by space charge in thevicinity of said emitter means,

b. said cathode means including first field forming electrode meansadjacent said emitter means for focusing emitted electrons into a beamin response to a potential on said first electrode means,

0. target anode means spaced from said emitter means and having asurface arranged to be impacted by said beam to produce x-radiation,said surface being at the equipotential and said emitter means being atthe 0% equipotential among equipotentials caused by applying a positivepotential to said target anode relative to said emitter means,

(I. second control elecrode means adjacent said first electrode meansand substantially closer thereto than to said 100% equipotential foraltering the electric field in the vicinity of said space charge suchthat additional electrons therefrom are made available for forming saidbeam and only a reduced amount of space charge remains when apredetermined potential is applied to said second electrode means thatis positive relative to said cathode means, said predetermined potentialalso establishing the width of said beam,

e. said second electrode means being the sole electrode between saidfield forming electrode means and said target anode means and comprisingan element having a surface disposed generally transversely to said beamand having an opening for passage of said beam, f. said last namedsurface and portions of the margins of said opening being other thanplanar and coinciding and conforming substantially with a selectedequipotential which is other than planar and has a value in the rangfe'of 1 t'o,1"5% or the potential difference between said target anodeand said cathodemeans,

g. application of said prede'ter'r nined positive potential to saidsecond electrode'means having a value equal substantially to the valueof said selected equipotential effectuating said reduced space chargeand maintaining the trajectories of said electrons in said beam tofollow substantially the path they wouldfollow in the absence of saidsecond electrode means and varying said predetermined potentialunsubstantially causing a change in the width of said beam with minoralteration of said space charge such that the intensity of said beam isstill limited and controlled primarily by the temperature of saidemitter means and is substantially independent of the potential on saidtarget anode.

2. The device set forth in claim 1 wherein:

a. said first electrode means has a plurality of holes therein,

b. said second electrode has a portion surrounding said first electrodemeans in spaced relation therewith, said second electrode means havingholes presented toward the holes in said first electrode means, and

c. insulating pin means extending from within said holes in said firstelectrode means to within holes in said second electrode means forsupporting said second electrode means from said first electrode means.

3. The invention set forth in claim 1 wherein:

a. said first field forming electrode means has at least two transverserecesses and oppositely diverging side walls defining the same forshaping said electric field,

b. said emission means comprising an elongated filament in each of saidrecesses.

4. The invention set forth in claim 1 wherein:

a. said second control electrode means has at least two transverserecesses and oppositely diverging side walls defining the same forshaping said electric field,

b. said emission means comprising an elongated filament in each of saidrecesses, and

0. rod means coextensive with said opening and intermediate the sidesthereof and between said emission means and disposed substantially onsaid selected equipotential.

5. The invention set forth in claim 1 wherein:

a. said second control electrode is located substantially coincidentwith an equipotential whose value is in the range of 1% to 15% of thepotential between said cathode and anode means.

6. An electron discharge device comprising:

a. an anode and cooperating cathode means,

b. said cathode means including a first field forming electrode meansand means for emitting electrons,

c. a second electrode means electrically isolated from and spaced fromsaid first electrode in the direction of said anode and disposedgenerally transversely to the path for an electron beam from said meansfor emitting electrons to said anode,

d. said second electrode means comprising a surface having an openingtherein which surface substantially conforms to a selected equipotentialthrough which electrons from said emitting means may pass with thetrajectories of said electrons being subi l stantially unaltered wherebywhen said second electrodemeans is energized with a positive potentialsubstantially equal to said equipotential it will "permit substantiallyunintercepted passage of electrons through said opening, i

e. said first electrode means having a cylindrical portion and saidsecond electrode means having a portion circumjacent to said firstelectrode means and in spaced relationship therewith, and

ff a plurality of insulating members substantially equiangilarly spacedaround said first electrode means and h aving corresponding ends engagedwith said first electrode means and opposed corresponding ends engagedwith said cylindrical portion of said second electrode means.

7. An x-ray generator comprising:

a. target anode means and a first electric field forming electrode meansspaced therefrom,

b. electron emission means proximate to said first electrode means forproviding a beam of electrons to impinge on said target anode means forproducing x-radiation,

c. second control electrode means interposed between said firstelectrode means and said target anode means and including a conductivemember disposed generally transversely to the path of said electron beamand having an opening therein, said member having a surface presentedtoward said target anode means,

d. said opening being defined by a margin surface which together withsaid surface of said member substantially conform with a selectedequipotential existing when there is a positive potential on said anodemeans relative to said first electrode means, said selectedequipotential being one across which electrons may pass with theirtrajectories being substantially unaltered, whereby said secondelectrode means is adapted to be energized with a positive potentialrelative to said second electrode means which positive potential issubstantially equal in magnitude to said selected equipotential suchthat it will permit substantially unintercepted passage of electronsthrough said opening,

e. said first field forming electrode means having at least two recesseseach of which has generally diverging sides,

f. said electron emission means being disposed in said recesses,respectively, and

g. a conductive rod means extending across the opening in said secondelectrode means and aligned intermediately of said recesses, said rodmeans being electrically connected with said second electrode means.

8. An x-ray generator comprising:

a. target anode means and a first electric field forming electrode meansspaced therefrom,

b. electron emission means proximate to said first electrode means forproviding a beam of electrons to impinge on said target anode means forproducing x-radiation,

c. second control electrode means interposed between said firstelectrode means and said target anode means and including a conductivemember disposed generally transversely to the path of said electron beamand having an opening therein, said member having a surface presentedtoward said target anode means,

d. said opening being defined by a margin surface e. said secondelectrode-means includes an annular part which is enclosed on one end bysaid transversely disposed member, said annular part surrounding saidfirst field forming electrode means and being in substantial concentricspaced relationship therewith,

a plurality of insulating pin means which each have corresponding endsengaged with said first electrode means and opposed corresponding endsengaged with said annular portion of said second electrode means forsupporting said second elec trode means from said first electrode means.

1. An x-ray generator comprising: a. cathode means including electronemitter means whose electron emission is limited and controlledprimarily by the selected temperature of said emitter means and whoseelectron emission available for forming an electron beam is subject tofurther limitation by space charge in the vicinity of said emittermeans, b. said cathode means including first field forming electrodemeans adjacent said emitter means for focusing emitted electrons into abeam in response to a potential on said first electrode means, c. targetanode means spaced from said emitTer means and having a surface arrangedto be impacted by said beam to produce xradiation, said surface being atthe 100% equipotential and said emitter means being at the 0%equipotential among equipotentials caused by applying a positivepotential to said target anode relative to said emitter means, d. secondcontrol elecrode means adjacent said first electrode means andsubstantially closer thereto than to said 100% equipotential foraltering the electric field in the vicinity of said space charge suchthat additional electrons therefrom are made available for forming saidbeam and only a reduced amount of space charge remains when apredetermined potential is applied to said second electrode means thatis positive relative to said cathode means, said predetermined potentialalso establishing the width of said beam, e. said second electrode meansbeing the sole electrode between said field forming electrode means andsaid target anode means and comprising an element having a surfacedisposed generally transversely to said beam and having an opening forpassage of said beam, f. said last named surface and portions of themargins of said opening being other than planar and coinciding andconforming substantially with a selected equipotential which is otherthan planar and has a value in the range of 1 to 15% of the potentialdifference between said target anode and said cathode means, g.application of said predetermined positive potential to said secondelectrode means having a value equal substantially to the value of saidselected equipotential effectuating said reduced space charge andmaintaining the trajectories of said electrons in said beam to followsubstantially the path they would follow in the absence of said secondelectrode means and varying said predetermined potential unsubstantiallycausing a change in the width of said beam with minor alteration of saidspace charge such that the intensity of said beam is still limited andcontrolled primarily by the temperature of said emitter means and issubstantially independent of the potential on said target anode.
 2. Thedevice set forth in claim 1 wherein: a. said first electrode means has aplurality of holes therein, b. said second electrode has a portionsurrounding said first electrode means in spaced relation therewith,said second electrode means having holes presented toward the holes insaid first electrode means, and c. insulating pin means extending fromwithin said holes in said first electrode means to within holes in saidsecond electrode means for supporting said second electrode means fromsaid first electrode means.
 3. The invention set forth in claim 1wherein: a. said first field forming electrode means has at least twotransverse recesses and oppositely diverging side walls defining thesame for shaping said electric field, b. said emission means comprisingan elongated filament in each of said recesses.
 4. The invention setforth in claim 1 wherein: a. said second control electrode means has atleast two transverse recesses and oppositely diverging side wallsdefining the same for shaping said electric field, b. said emissionmeans comprising an elongated filament in each of said recesses, and c.rod means coextensive with said opening and intermediate the sidesthereof and between said emission means and disposed substantially onsaid selected equipotential.
 5. The invention set forth in claim 1wherein: a. said second control electrode is located substantiallycoincident with an equipotential whose value is in the range of 1% to15% of the potential between said cathode and anode means.
 6. Anelectron discharge device comprising: a. an anode and cooperatingcathode means, b. said cathode means including a first field formingelectrode means and means for emitting electrons, c. a second electrodemeans electrically isolated from and spaced from said fiRst electrode inthe direction of said anode and disposed generally transversely to thepath for an electron beam from said means for emitting electrons to saidanode, d. said second electrode means comprising a surface having anopening therein which surface substantially conforms to a selectedequipotential through which electrons from said emitting means may passwith the trajectories of said electrons being substantially unalteredwhereby when said second electrode means is energized with a positivepotential substantially equal to said equipotential it will permitsubstantially unintercepted passage of electrons through said opening,e. said first electrode means having a cylindrical portion and saidsecond electrode means having a portion circumjacent to said firstelectrode means and in spaced relationship therewith, and f. a pluralityof insulating members substantially equiangularly spaced around saidfirst electrode means and having corresponding ends engaged with saidfirst electrode means and opposed corresponding ends engaged with saidcylindrical portion of said second electrode means.
 7. An x-raygenerator comprising: a. target anode means and a first electric fieldforming electrode means spaced therefrom, b. electron emission meansproximate to said first electrode means for providing a beam ofelectrons to impinge on said target anode means for producingx-radiation, c. second control electrode means interposed between saidfirst electrode means and said target anode means and including aconductive member disposed generally transversely to the path of saidelectron beam and having an opening therein, said member having asurface presented toward said target anode means, d. said opening beingdefined by a margin surface which together with said surface of saidmember substantially conform with a selected equipotential existing whenthere is a positive potential on said anode means relative to said firstelectrode means, said selected equipotential being one across whichelectrons may pass with their trajectories being substantiallyunaltered, whereby said second electrode means is adapted to beenergized with a positive potential relative to said second electrodemeans which positive potential is substantially equal in magnitude tosaid selected equipotential such that it will permit substantiallyunintercepted passage of electrons through said opening, e. said firstfield forming electrode means having at least two recesses each of whichhas generally diverging sides, f. said electron emission means beingdisposed in said recesses, respectively, and g. a conductive rod meansextending across the opening in said second electrode means and alignedintermediately of said recesses, said rod means being electricallyconnected with said second electrode means.
 8. An x-ray generatorcomprising: a. target anode means and a first electric field formingelectrode means spaced therefrom, b. electron emission means proximateto said first electrode means for providing a beam of electrons toimpinge on said target anode means for producing x-radiation, c. secondcontrol electrode means interposed between said first electrode meansand said target anode means and including a conductive member disposedgenerally transversely to the path of said electron beam and having anopening therein, said member having a surface presented toward saidtarget anode means, d. said opening being defined by a margin surfacewhich together with said surface of said member substantially conformwith a selected equipotential existing when there is a positivepotential on said anode means relative to said first electrode means,said selected equipotential being one across which electrons may passwith their trajectories being substantially unaltered, whereby saidsecond electrode means is adapted to be energized with a positivepotential relative to said second electrode means which positivepotential is substAntially equal in magnitude to said selectedequipotential such that it will permit substantially uninterceptedpassage of electrons through said opening, e. said second electrodemeans includes an annular part which is enclosed on one end by saidtransversely disposed member, said annular part surrounding said firstfield forming electrode means and being in substantial concentric spacedrelationship therewith, f. a plurality of insulating pin means whicheach have corresponding ends engaged with said first electrode means andopposed corresponding ends engaged with said annular portion of saidsecond electrode means for supporting said second electrode means fromsaid first electrode means.